Progress in the production and application of n-butanol as a biofuel

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Abstract

Butanol is a very competitive renewable biofuel for use in internal combustion engines given its many advantages. In this review, the properties of butanol are compared with the conventional gasoline, diesel fuel, and some widely used biofuels, i.e. methanol, ethanol, biodiesel. The comparison of fuel properties indicates that n-butanol has the potential to overcome the drawbacks brought by low-carbon alcohols or biodiesel. Then, the development of butanol production is reviewed and various methods for increasing fermentative butanol production are introduced in detailed, i.e. metabolic engineering of the Clostridia, advanced fermentation technique. The most costive part of the fermentation is the substrate, so methods involved in renewed substrates are also mentioned. Next, the applications of butanol as a biofuel are summarized from three aspects: (1) fundamental combustion experiments in some well-defined burning reactors; (2) a substitute for gasoline in spark ignition engine; (3) a substitute for diesel fuel in compression ignition engine. These studies demonstrate that butanol, as a potential second generation biofuel, is a better alternative for the gasoline or diesel fuel, from the viewpoints of combustion characteristics, engine performance, and exhaust emissions. However, butanol has not been intensively studied when compared to ethanol or biodiesel, for which considerable numbers of reports are available. Finally, some challenges and future research directions are outlined in the last section of this review.

Introduction

Biofuels are receiving increasing public and scientific attention, driven by factors such as uncertainties related to oil price, greenhouse gas emission, and the need for increased energy security and diversity. Biofuels are a wide range of fuels which are in some way derived from biomass [1]. It is reported that fossil fuels – oil, coal, and natural gas – dominated the world energy economy, covering more than 80% of the total primary energy supply [2]. Renewable energy sources accounted for 9.8% of the world's total primary energy supply in 2007, as shown in Table 1. Even for the 9.8% renewable energy, approximately two-thirds of biomass was used for cooking and heating because of its widespread noncommercial use in developing countries. The remaining one-third of biomass energy was utilized both in industrial applications within the heat, power, and road transportation sectors and for heating purposes in the private sector in industrialized countries. Actually, only about 2% in total global transport consumption was from the biofuels produced from biomass. Therefore, there is a large potential for the biofuels in the transportation and some other energy supply area.

The share of biofuels in the automotive fuel market is expected to grow rapidly in the next decade. In the USA, the environment protection agency renewable fuel standard version 2 (EPA-RFS2) and the Californial low-carbon fuel standard are driving the US market. The EPA-RFS2 requires that 36 billion gallons (136 billion liter) of renewable fuel should be available in the US market by 2022 [3]. For the transport sector of the European Union, 10% transportation fuel from biofuel is targeted by 2020 [4]. In addition, biofuels also have great potential in some other countries, such as Brazil, China, etc. in the next 10 years [5], [6].

For the transportation vehicles, various biofuels have been researched or applied, such as biodiesel, bioDME, biomethanol, bioethanol, biobutanol, etc. All these biofuels can be derived from renewable feedstock as opposed to from fossil feedstock in the case of gasoline or diesel fuels. One widely used biofuel is biodiesel, which is defined as the mono-alkyl esters of long chain fatty acids derived from renewable feedstock, such as vegetable oil, animal fats, algae, etc. Biodiesel, considered as a possible substitute of conventional diesel fuel, usually consists of fatty acid methyl/ethyl esters, obtained from triglycerides by transesterification with methanol/ethanol respectively [7], [8], [9], [10]. Biodiesel has many similar properties like the diesel fuel and it can blend with convention diesel in any proportion. Many researchers like Qin et al. [11], [12], Huang et al. [13], Fang and Lee [14], [15], Qi et al. [16], Bhale et al. [17] have reported that power output of biodiesel was almost identical to that of diesel, and the soot, carbon monoxide (CO), carbon dioxide (CO2) and total hydrocarbon (THC) emissions are reduced in biodiesel and its blends, because of its oxygen content which leads to more complete combustion. However, the NOx emission is reported to be in the range between ±10% as compared to diesel depending on engines combustion characteristics [8]. In addition, biodiesel also has many other advantages in comparison to diesel fuel, such as non-toxic, easy degradation, more safe due to higher flash point, more clean combustion, 90% reduction in cancer risks, lower polycyclic aromatic hydrocarbon (PAH) and nitro PAH compounds emissions. However, there are still some disadvantages, such as difficulty in storage due to an easier degeneration, the decline of flow characteristics at low fuel temperatures, and more expensive due to less production.

The other widely used biofuels are bioalcohols. Alcohols, mainly ethanol and to a much lesser extent methanol, which are considered as alternative fuels for internal combustion engines [18], [19], [20], [21], [22], [23]. Ethanol is a biomass-based renewable fuel that can be produced by alcoholic fermentation of sugar from vegetable materials, such as corn, sugar cane, sugar beets, barley, sweet sorghum, and agricultural residues [23], [24], [25], but methanol is mainly produced from coal or petrol based fuels. Therefore, ethanol is superior to methanol due to its renewability and is widely used as an additive or alternative fuel in many countries, such as the United States, Brazil, China, etc. However, there are also several critical issues that need to be considered with the use of ethanol as an engine fuel [25], [26], [27], [28]. Ethanol is corrosive to the existing pipelines through general corrosion, dry corrosion and wet corrosion. General corrosion is caused by ionic impurities, mainly chloride ions and acetic acid. Dry corrosion is attributed to the ethanol molecule and its polarity. Some metals, such as magnesium, lead and aluminum are susceptible to chemical attack by dry ethanol. Wet corrosion is caused by the ethanol to absorb moisture from the atmosphere, which oxidizes most metals. And it may tend to be more corrosive as it passes through the fuel injection system. Further, non-metallic components have also been affected by ethanol with particular reference to elastomeric components. Detailed effects of corrosiveness have been reviewed by Hansen et al. [23]. Ethanol has much lower flash point than the diesel fuel and has higher vapor formation potential in confined spaces, thus requiring extra caution in its usage. Some surfactants or cosolvents must be used in order to ensure solubility of ethanol and diesel fuel.

A very competitive biofuel for use in engines is butanol. Like ethanol, butanol is a biomass-based renewable fuel that can be produced by alcoholic fermentation of the biomass feedstocks [25], [29], [30], [31]. Butanol has a 4-carbon structure and is a more complex alcohol than methanol and ethanol that only has 1 and 2-carbon structure respectively. Butanol, like ethanol, can blend with gasoline very well. Furthermore, butanol could be a future option for blending with diesel. Butanol contains more oxygen content compared with the biodiesel, leading to further reduction of soot. NOx emissions can also be reduced due to its higher heat of evaporation, which results in a lower combustion temperature [32]. Therefore, the butanol has more advantages than the widely used ethanol and biodiesel. However, the main disadvantage of butanol is its quite low production. Compared butanol yield by acetone butanol ethanol (ABE) fermentation to that of the yeast ethanol fermentation process, the yeast process yields of ethanol has a 10–30 times higher production rate. This becomes very clear why ethanol was chosen as an alternative fuel source over butanol during the oil crisis in the 1970s and 1980s. However, with the development of butanol fermentative process, a higher butanol production rate has become possible, explaining the increasing studies on it in recent years.

In this review, the properties of butanol are compared with those of the conventional gasoline, diesel fuel, and some widely used biofuels, i.e. methanol, ethanol, biodiesel. Then, the development of butanol production is reviewed and various methods for increasing the production of butanol are introduced in detailed. Next, the applications of butanol as a biofuel are summarized from three aspects. First, the fundamental combustion experiments in some burning reactors are reviewed, including the progress in the chemical kinetics. Second, the applications of butanol in SI engines, including the studies on cooperative fuel research engines. Third, the applications of butanol in CI engines, including the studies on properties of butanol–diesel blends and some new combustion technologies, i.e. homogeneous charge compression ignition (HCCI), low temperature combustion (LTC). Finally, some challenges and future research directions are outlined in this paper.

Section snippets

Properties of butanol isomers

Alcohols are defined by the presence of a hydroxyl group (–OH) attached to one of the carbon atoms. Butanol has a 4-carbon structure and the carbon atoms can form either a straight-chain or a branched structure, resulting in different properties. There exist different isomers, based on the location of the –OH and carbon chain structure. The molecular structure and the main applications of butanol isomers are listed in Table 2. 1-butanol, also better known as n-butanol, has a straight-chain

The history of n-butanol production

The discovery of n-butanol as a regular constituent of fusel oil was achieved by Wirtz in 1852. Ten years later, 1862, Pasteur, from his experiments, concluded that butyl alcohol was a direct product of anaerobic conversion of lactic acid and calcium lactate. Between 1876 and 1910, several scholars researched the production of acetone–butanol and related solvents [38]. Production of industrial butanol and acetone, via ABE (acetone, butanol, ethanol) fermentation started in 1912–1916 [39], [40],

Progress in applications of n-butanol as a biofuel

As aforementioned, n-butanol has the potential to overcome the drawbacks brought by low-carbon alcohols, and many new methods have increased the production of n-butanol. On the other hand, some factors, such as uncertain oil price, greenhouse gas emission, and the need for increased energy security and diversity, promote the development of biofuels. Thus, investigations of n-butanol as an alternative biofuel have been conducted by several research groups recently, in which butanol has been

Summary and conclusions

In this review, the properties of butanol are compared with those of conventional gasoline and diesel fuel and some widely used biofuels, i.e. methanol, ethanol, biodiesel, which indicates that n-butanol has the potential to overcome the drawbacks brought by low-carbon alcohols in several aspects. The main advantages of n-butanol include higher heating value, lower volatility, less ignition problems, good intersolubility with diesel without any cosolvents, more suitable viscosity as a

Acknowledgement

The authors would like to appreciate Xuan Feng from University of Illinois at Urbana-Champaign for his proofreading and English revision of this manuscript. This work was funded by the National Natural Science Found of China (50976078) and the Ministry of Science and Technology of China (2007CB210002). The author Haifeng Liu also thanks the China Scholarship Council for providing a research scholarship in the USA throughout the project [2009]3012.

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